1. Field of the Invention
The present invention relates generally to optical networking, and more particularly, to a method and apparatus for optical swapping of digitally-encoded label information within an optical stream or packet.
2. Introduction
Packet networks transport data from a source location to a destination location by organizing the data into self-contained units called packets. Each packet carries its routing information as it passes through a series of routing nodes on its way to the destination location. Each routing node reads the routing information associated with the packet and uses that information to decide the correct path to use to forward the packet. In traditional IP (Internet Protocol) networks, the routing information is made up of individual addresses of source and destination nodes. In more advanced MPLS (Multiprotocol Label Switching) networks, packets are assigned additional labels that group them according to their intermediate or final destinations. This label assignment promotes efficient scaling and quality-of-service assignment in the MPLS networks. In both IP and MPLS networks with optical transport between routing nodes, all packets are converted from optical to electrical form as they enter the routing node and then converted back to optical form as they leave the routing node. Minimization of such Optical-Electronic-Optical (OEO) conversions is a central principle of cost reduction in advanced optical networks. Removing OEO conversions and electronic switching also helps with scaling the routers to massive capacity, since these electronic functions contribute to the buildup of cost, failure rate, and power dissipation in the router nodes.
As a result, Optical Label Switching (OLS) networks are under intensive study as a means of combining the flexibility and statistical multiplexing of electronic IP packet networks with the cost-effectiveness and massive scalability of optical data transport. OLS networks include an optical label (OL) with each packet of payload data, and the OL is read at each routing node to determine the proper switch settings for packet forwarding. OLs may be in-band, sent as headers occupying the first bytes of every packet, but that approach requires expensive photoreceivers capable of operating at full data rate. Thus, OLs are usually sent in a separate out-of-band channel. For flexibility and scaling, it is desirable to be able to change a value of an optical label as it passes through a routing node. This is known as label swapping.
The value of OLS networks is greatly enhanced when they can carry multiple optical packets simultaneously. Such a capability can be implemented through the use of wavelength division multiplexing (WDM), in which each packet is assigned to a specific wavelength of light. Using WDM, multiple simultaneous packets are combined at the source with an arrangement of optical wavelength filters called a wavelength multiplexer (MUX), and re-separated before detection at the destination with a reciprocal arrangement of optical filters called a wavelength demultiplexer (DMUX). WDM presents OLS networks with an additional challenge of reading multiple OLs that are simultaneously present at any given point in the network. For in-band labels, the straightforward solution is to follow a DMUX with a parallel array of label receivers, but this becomes expensive as the wavelength count becomes large. Alternatively, one might place a tunable wavelength selection filter before a shared label receiver, but this would reduce packet throughput and demand extremely complex network synchronization.
Various of out-of-band OL technologies have been proposed for WDM OLS networks. Some use dedicated label wavelengths for each packet wavelength, reducing spectral efficiency of the networks. Others rely on orthogonal modulation formats, such as optical phase shift keying (PSK) for labels in combination with amplitude shift keying (ASK) for payloads. Although this approach can reduce the number of full-rate OEO conversions and enable optical label switching, it often has flaws such as complex modulation formats, crosstalk caused by optical impairments, or high cost.
Another key element of optical packet networks is the all-optical regenerator. As packets pass through optical transmission lines that may be hundreds or thousands of km long, they accumulate impairments that degrade the quality of the pulses that represent data bits. If not corrected, these impairments will cause bit errors and corrupt the packets. Regenerators are non-linear signal processing subsystems that restore the correct amplitude, pulse shape, and timing to each bit of the packet. To minimize OEO conversions, all-optical regenerators have been developed, and used to demonstrate error-free data transmission up to 1 million km. All-optical regenerators can be classed as non-inverting or inverting, depending on whether or not 1s are exchanged with 0s in the regeneration process. Because the inversion affects all bits in exactly the same way, the original data can be easily recovered by an additional inversion process at the destination.
Optical labels can also be useful in circuit-switched networks, especially those capable of dynamically re-routing signals on a wavelength-by-wavelength basis.
Thus, there is a need for a practical method of encoding optical labels carrying routing information or other information in optical packet networks and also for a method of optical label swapping applicable to both packet-switched and circuit-switched optical networks.